Fetal membranes and placentation in Chiroptera A. Gopalakrishna and K. B. Karim Department ofZoology, Institute of Science, Nagpur, India Introduction The development of the fetal membranes and the definitive structure of the placenta have been studied in one or a few representatives of the following families of Chiroptera: Pteropidae (Gohre, 1892a, b; Selenka, 1892; Kohlbrugge, 1913; Keibel, 1922; van der Sprenkel, 1932; Moghe, 1951, 1956; Wimsatt, 1958; Karim, 1971, 1972a), Rhinopomatidae (Srivastava, 1952; Gopalakrishna, 1958), Emballonuridae (Gopalakrishna, 1958; Wimsatt & Gopalakrishna, 1958), Noctilionidae (Anderson & Wimsatt, 1953, 1963), Megadermatidae (Gopalakrishna, 1950b, 1958; Gopalakrishna & Khaparde, 1978a, b), Rhinolophidae (Hamlett, 1934; Gopalakrishna & Bhiwgade, 1974; Bhiwgade, 1976, 1977), Hipposideridae (Gopalakrishna, 1958; Gopalakrishna & Moghe, 1960; Jeevaji, 1973), Phyllostomatidae (Hamlett, 1935; Wislocki & Fawcett, 1941; Fleming, 1971; Rasweiler, 1972, 1974; Bodley, 1974; Bleier, 1975), Desmodontidae (Wimsatt, 1954; Bjorkman & Wimsatt, 1968; Quintero & Rasweiler, 1974), Vespertilionidae (several early authors and, more recently, Wimsatt, 1944, 1945, 1958; Gopalakrishna, 1949, 1950a, 1958; Enders & Wimsatt, 1968; Potts & Racey, 1971; Phansalkar, 1972; Sapkal, 1973) and Molossidae (Hamlett, 1934; Mossman, 1937; Stephens, 1962, 1969; Stephens & Easterbrook, 1968, 1969, 1971; Stephens & Cabral, 1971). These studies have revealed that there are considerable differences among the members of the different families, and sometimes among members of the same family, in the details of the development of the fetal membranes and the final structure of the placenta. Observations on the development of fetal membranes and placentation made by a group of workers in this laboratory during the past few years, and much of which is yet unpublished, are incorporated in the present review. The fetal membranes and the structure of the placenta of Rousettus leschenaulti, Taphozous melanopogon, Hipposideros speoris, H. lankadiva, Pipistrellus ceylonicus chrysothrix, P. dormeri, P. m. mimus, and Tadarida aegyptiaca are described for the first time, and additional information or corrections of earlier erroneous observations on Cynopterus sphinx gangeticus, Pteropus giganteus, Megaderma lyra lyra, Hipposiderosfulvus, and Miniopterus schreibersii fluiginosus are also included here. Amnion Four modes of amniogenesis are noticed among bats. (1) The early amniotic cavity arises by the breakdown and degeneration of some of the cells in the centre of the embryonic mass, and, as the embryonic plate expands, the roof of the amniotic cavity becomes stretched to form a thin layer which persists as the ectodermal component of the amnion over which extends extra-embryonic mesoderm. Such a method of amnion formation occurs in Rousettus leschenaulti, Pteropus giganteus and Scotophilus heathi, and was reported earlier in Glossophaga soricina (Hamlett, 1935), Scotophilus wroughtoni (Gopalakrishna, 1949) and Desmodus rotundus murinus (Wimsatt, 1954). (2) In Megaderma I. lyra, Hipposideros speoris, H. lankadiva and all the pipistrellids studied here, the primitive amniotic cavity is formed as in the species described above, but, with the expansion of the embryonic plate and the primitive amniotic cavity, the original roof of the Downloaded from Bioscientifica.com at 10/06/2021 06:56:34PM via free access primitive amniotic cavity ruptures and is lost. Hence, for a short time the amniotic cavity is roofed over by the basal layer of trophoblast. However, folds, which soon develop from the sides of the embryonic plate, grow dorsally and arch over the amniotic cavity and form the ectodermal component of the definitive amnion. Extra-embryonic mesoderm envelops this layer to establish the definitive amnion. Such a method of amnion formation had been reported earlier in Vespertilio murinus (Duval, 1894, 1895, 1896), Vesperugo noctula (van der Stricht, 1899), Miniopterus schreibersii (Celestino da Costa, 1920), Vesperugo leisleri (Ramaswami, 1933), and Myotis lucifugus lucifugus (Wimsatt, 1944). (3) The amnion develops in an unusual manner in Cynopterus sphinx gangeticus (Moghe, 1956) and possibly in C. marginatus (Keibel, 1922). In these species, after the primitive amniotic cavity is formed by the degeneration of some of the cells of the embryonic mass, the embryonic mass is pushed deeper into the blastocyst cavity by a proliferation of precociously formed extra- embryonic mesoderm. Thus, a mass of spongy tissue intervenes between the hollow embryonic mass and the trophoblast layer. Folds developing from the sides of the embryonic mass grow dorsally and arch over, enclosing a cavity in the spongy tissue. For a short time, therefore, there are two cavities lying over one another—the original primitive amniotic cavity and the secondary amniotic cavity—separated by the roof of the primitive amniotic cavity. With the expansion of the embryonic plate the partition between the two cavities ruptures and they become confluent. The roof of the secondary amniotic cavity forms the ectodermal component of the definitive amnion after it is encompassed by the extra-embryonic mesoderm (Text-fig. 1). (4) The amnion in Tadarida brasiliensis cynocephala (Stephens, 1962) forms exclusively by the development of folds. It is interesting to note here that not only is this the sole bat in which this method of amniogenesis has been reported, but it is also the only species in which the blastocyst establishes its first contact with the uterine wall by its abembryonic pole. In Molossus Text-fig. 1. Schematic representation of the development (a-f) of the amnion in Cynopterus sphinx gangeticus. am, amnion; mes, mesoderm; p.am.c, primitive amniotic cavity; s.am.c, secondary amniotic cavity; y-s.c, yolk-sac cavity. Downloaded from Bioscientifica.com at 10/06/2021 06:56:34PM via free access rufus and M. obscurus (Sansom, 1932), in which early implanted blastocysts have been described, the blastocyst comes into contact with the uterine wall on its entire surface, and the embryonic mass has a central primitive amniotic cavity. From the above, it appears that the mode of formation of the amnion, whether by cavitation or by folding, is determined by the topographical relationship between the uterus and the im¬ planted blastocyst. Where the blastocyst is attached either at its embryonic pole as in Pteropus giganteus (Moghe, 1951) and vespertilionids (Wimsatt, 1944; Gopalakrishna, 1949) or circumferentially as in Rousettus leschenaulti, Megaderma I. lyra, Hipposideros speoris, Miniop¬ terus schreibersii fuliginosus, the primitive amniotic cavity develops by cavitation as there is no space for the development of the folds. On the other hand, in Tadarida brasiliensis cynocephala, in which the embryonic mass hangs freely into the uterine lumen, amniogenesis takes place by fold formation since space is available for the development of the folds. Yolk sac The developmental history of the yolk sac in bats follows three distinct courses with minor variations. (1) The simplest course of development is illustrated by Noctilio labialis minor (Anderson & Wimsatt, 1963), Megaderma I. lyra, Desmodus rotundus (Wimsatt, 1954), Artibeus jamaicensis parvipes (Wislocki & Fawcett, 1941) and all vespertilionids (Wimsatt, 1945; Gopalakrishna, 1950a) in which the abembryonic segment of the yolk sac remains permanently bilaminar or trilaminar either in contact with the uterine wall, as in Noctilio labialis minor, Megaderma I. lyra, Artibeus jamaicensis parvipes, Desmodus rotundus murinus and Miniopterus schreibersii fuliginosus, or remains hanging freely in the uterine lumen as in all other vespertilionids. The vascular splanchnopleure becomes progressively 'invaginated' (Wimsatt, 1945) towards the abembryonic segment of the yolk-sac wall. As the vitelline vessels extend from the embryonic to the abembryonic pole, the endodermal cells, which were squamous to begin with, become cuboidal (PI. 1, Fig. 1), and the separation of the splanchnopleure is accompanied by a further hypertrophy of the endodermal cells. In the case of Megaderma I. lyra and all the vespertilionids, as the yolk sac splanchnopleure pushes towards the abembryonic yolk-sac wall, it becomes folded and the mesodermal cells also undergo enormous hypertrophy. During the final stages of gestation the splanchnopleure undergoes almost complete collapse resulting in the near obliteration of the yolk-sac lumen which persists only in some places as narrow streak-like spaces (PI. 1, Fig. 2). Whereas the endoderm in these bats remains as a single layer of cuboidal cells lining the remnants of the yolk-sac lumen, the mesodermal elements occur as numerous folds of large vacuolated cells projecting into the exocoelom (PI. 1, Fig. 3). The enormous hypertrophy of the cellular elements of the collapsed splanchnopleure gives it a gland-like appearance. The yolk sac of Tadarida brasiliensis cynocephala (Stephens, 1962) and T. aegyptiaca appears to undergo the highest specialization among bats belonging to this category, since in these species the close approximation of the collapsed walls of the yolk sac and the hypertrophy of the endodermal cells nearly obliterates the yolk-sac lumen (PI. 1, Figs. 4 and 5). (2) The second mode of vitellogenesis is illustrated by Rhinopoma kinneari (Srivastava, 1952; Gopalakrishna, 1958), Taphozous longimanus (Gopalakrishna, 1958), T. melanopogon, Rhinolophus rouxi (Bhiwgade, 1977), Hipposideros bicolor pallidus (Gopalakrishna,